1. Field of the Invention
The present invention relates generally to internal combustion engines, and in particular to a system for driving engine accessories with exhaust gas.
2. Description of the Related Art
The output power of an automotive engine is used not only to propel the vehicle having the engine mounted therein but also to drive engine subsystems or accessories, such as an alternator, a water pump, the compressor of an air conditioner, power steering, power brakes, pollution control pumps, and the like. Such subsystems and accessories are conventionally driven from the engine shaft, typically by pulleys and belts engaged therebetween. Specialty types of vehicles often have other types of devices which are driven from the power of their engines. Large trucks usually have engine driven compressors to provide compressed air for air brakes. Tow trucks usually have hydraulic pumps to drive winches and hydraulic lift cylinders. Agricultural tractors often have power take-off (PTO) shafts geared from the tractor transmission as well as hydraulic pumps. Construction and excavation vehicles employ hydraulic pumps to drive hydraulic cylinders for the operating devices thereof. The engine power required to drive the accessories of the engine is not available for propelling the vehicle having the engine mounted therein.
One of the problems associated with engine and vehicle accessories which are belt driven from the engine crankshaft is uneven wear of the front and rear main bearings of the engine. The tension in the belt engaged with the main pulley or harmonic balancer/pulley combination usually pulls the front end of the shaft upward, which is reacted to by the rear end of the crankshaft pushing downward on the main rear bearing. The problem is exacerbated if the belts are over-tensioned. Such radial forces on the crankshaft ends accelerate bearing wear which can result in oil leakage through the worn bearings as clearances are enlarged and the circulation of ground bearing materials throughout the engine which increases engine wear and increases the tendency of some oil passages to clog.
Another problem with belt driven accessories and accessories which are otherwise driven directly from the crankshaft is that the accessories must be clustered around and in close proximity to the engine. The result is that the engine compartments of modern vehicles are extremely crowded, making access for maintenance and repair difficult.
Some engine subsystems and accessories are vital to the operation of the engine or the vehicle and are required to be operating or available for use whenever the engine is running. Other accessories are used relatively infrequently and may be effectively disengaged when not in use, by means such as a solenoid operated clutch or the like. Typically, belt driven accessories are driven at a speed which is related to the rotational speed of the engine crankshaft. Further control is usually accomplished in an on/off manner. For example, the alternator is cycled on and off by an internal voltage regulator. The air conditioner compressor has a solenoid clutch which is thermostatically controlled in an on/off manner. Coolant circulation is controlled by a thermostatic valve which bypasses the radiator unless the coolant temperature exceeds the set point of the thermostat. Radiator fans on modern automobiles are often rotated by a thermostatically controlled electric motor, instead of being belt driven from the engine crankshaft as was formerly done.
Conventional automotive engines are not notably energy efficient. Combustion of the fuel, usually gasoline, is incomplete with a small proportion of the consumed fuel being exhausted to the atmosphere uncombusted and a somewhat larger proportion being exhausted to the atmosphere in an incompletely combusted state as carbon monoxide. Of the energy which is derived from the combustion of the fuel, a large proportion is wasted without doing useful work as heated exhaust gases.
In response to the decreasing supplies of crude oil and the resulting rise in prices of automotive fuel and the increasing concerns regarding atmospheric pollution caused by automotive emissions, efforts have been directed to increasing the fuel efficiency and reducing the emissions of automobile engines. Accordingly, passenger cars and engines therefor have been reduced in size, or "downsized". One result of such downsizing is that some aspects of performance, such as start-off and passing acceleration, suffer. Acceleration performance in an automobile is not merely a recreational aspect of driving, but is also a safety concern, particularly in safely entering high speed, congested traffic without disrupting the traffic flow.
In order to increase their acceleration performance, some cars are equipped with turbo-superchargers or turbochargers which supercharge the engine during hard acceleration. A turbocharger includes a turbine driven by exhaust gases and connected to a compressor feeding air to the intake of the engine at superatmospheric pressure to increase the power output from the engine. The communication of exhaust gases to the turbine is controlled by a waste gate in the exhaust system which is often controlled by engine vacuum or which may be linked to the accelerator pedal such that the waste gate is opened to communicate the exhaust gases to the turbine when the accelerator is pressed past a given point. Otherwise, the turbine is bypassed by the waste gate whereby the engine normally operates in a nonsupercharged mode for economy. Many large diesel trucks employ exhaust driven turbochargers in combination with belt driven superchargers to increase the performance of an engine of a given size rather than employing a larger engine.